More and more, the cellular service providers are focusing on techniques for high-capacity and efficient communication of digital information over wireless links. In 1998 the Chinese Wireless Telecommunications Standards proposed to the International Communications Union a new standard that is based on Time Division Duplexing (TDD) and Synchronous Code-Division Multiple Access (CDMA) technology (TD-SCDMA) for TDD. The International Communications Union has approved and adopted this proposal. The TDD uses a single frequency band for uplink as well as downlink, but at different predetermined time slots. The CDMA is based on Direct-Sequence Spread-Spectrum (DS-SS) principles, where multiple users simultaneously occupy the same radio frequency channel, separated only by user-specific spreading or signature sequences.
DS-SS communication requires detection of one or more spreading chip-code sequences embedded in an incoming spread-spectrum signal as well as subsequent synchronization of the receiver with the detected chip-code sequence. Also, prior to transmission, predetermined symbols (e.g., midambles) are inserted in each frame to detect and compensate for the distortion of the information symbols by comparing their distortion to the distortion of the predetermined symbols. In other words a transmitter inserts what are called training symbols in each frame, and a receiver, which already expects these training symbols, extracts the distorted symbols from the received frames and uses their distortion information for channel estimation. As a result, in TD-SCDMA systems, time slots and spreading codes separate the users in a cell.
In a CDMA environment, as well as other types of communication settings employing DS-SS, two or more transmitters may transmit at the same time using different spreading codes. The individual channels interfere with one another, since the characteristics of the spreading codes that are used are not ideal. In such a situation, particularly if the receiver must receive the transmissions simultaneously, the receiver must search for and acquire multiple codes at the same time from within a broad-spectrum wireless signal.
In a CDMA system, the multiple access interference (MAI) affects all users equally. While detection schemes such as the rake receiver are sub-optimal because they only consider the user's signal information without any attempt to characterize the interference from other users, the Joint Detection algorithms process all users in parallel and include the interference information from all. Joint detection and its associated parallel processing are well suited for TD-SCDMA systems because in every time slot the users are synchronized and are limited to a very manageable number. The result is a joint detector of reasonable complexity that can easily be implemented in today's parallel computational architectures.
In addition to the MAI problem, because the signal transmitted by a wireless terminal to a base station is radiated omnidirectionally from the wireless terminal, some of the transmitted signal may reach the base station in a direct, line-of-sight path, while most of the transmitted signal radiates in other directions and never reaches the base station. Hence, some of the signals that radiate initially in a direction other than toward the base station strike an object, such as a building, and are reflected toward the base station. Therefore, a signal can radiate from the wireless terminal and be received by the base station via multiple signal paths. Such a signal and its reflections arrive at the base station at different times and will interfere to form a composite of several constituent signals. This is known as “multipath” interference. Furthermore, the characteristics of each received signal are affected by the length of the path traveled and the objects from which the signal has been reflected.
Furthermore, for a CDMA system to operate at all and to allow the available frequency range to be used optimally, it is of major importance to have the same interference power magnitude, at the receiver, on each individual channel. Otherwise, it is possible for a channel with a comparatively high interference power to conceal the other channels, and to make their detection impossible. For this reason, every CDMA system uses power control. In general, power control in a CDMA system plays a major role, with a critical influence on the overall performance of the system.
The power control per se is specified by the respective standard. For a CDMA system, the power control is based on measuring the SINR, which is the ratio of the useful power to the interference power in a detected channel. The receiver then transmits this measured value in the form of a transmission power control command (TCP) back to the transmitter on the back channel. The transmitter then individually adapts the transmission power for each channel, in order to achieve a standard SINR for all the channels in the receiver. One advantageous side effect in this case is that this power control can compensate within certain limits for fluctuations in the physical mobile radio channel (slow fading), thus allowing the transmission capacity to be increased.
The measurement of the useful power is relatively simple; however, it is considerably more difficult to measure the interference power, although this has a significant influence on the measurement accuracy of the SINR, since this factor is located in the denominator of the useful power to interference power ratio. The UMTS (Universal Mobile Telecommunication System) Standard states that the interference power should be determined from the pilot symbols, which are known apriori to the receiver, after the despreading of the received signal.
Almost every stage of the many stages of the transmission and the reception of a signal, such as spreading, despreading, filtering, and Joint Detection, requires numerous computations. These computations, in addition to the received signal, utilize various radio-related quantities, where measurement of each quantity necessitates further computations. Simplification of any required computation will directly affect a radio network's speed and efficiency and, as a result, its capacity.